88 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 18, 



the lake. The waves then spread out the materials along the shore, 

 and throw some of them upon the beach, their dispersing power 

 being aided by the ice, which often adheres to pebbles during the 

 winter, and gives buoyancy to them '^ *. 



Influenced by this idea of matter being "thrown up" by the 

 waves, he accounts for the intermediate shelf on Tombhran by the 

 fact that " it occurs where there was the longest space of open 

 water, and where the waves may have acquired a more than ordi- 

 nary power to heap up detritus ". 



It is fair to remember that aU these eminent writers were prin- 

 cipally occupied in considering how the water filled the valley. If 

 their attention had been mainly directed to the manner in which 

 the shelves had been formed, I cannot but think that they would 

 have arrived at a different explanation. 



Fig. 2. — Section of the Roads Fig. 3. — Section of the Roads ac- 



according to the theory of cording to Sir T. Lauder 



Macculloch. DicWs theory. 



If, for instance, the theory advocated by Sir T. L. Dick and Prof. 

 Rogers were sufficient to explain the whole matter, the section pre- 

 sented by the side of the hill ought to have shown an excavation, as in 

 fig. 3. On the other hand, if the theory first proposed by Dr. Mac- 

 crdloch and adopted by Mr. Darwin, Mr. Milne, Mr. Jamieson, and 

 Sir Charles Lyell were the correct one, the section would be a projec- 

 tion, as in fig. 2. The true outline, however, as admitted by almost 

 all observers, is that represented in fig. 1 ; nowhere, as we have 

 already seen, does it resemble that of fig. 2. Moreover, if the roads 

 were owing to the heaping up of loose detritus "washed down 

 annually, and especially during the melting of snow," the shelves 

 ought to be broadest where rivulets come down the sides of the 

 hills, whereas, on the contrary, these streamlets have, as is well 

 known, actually produced the opposite effect, and cut back the hill- 

 side. 



Not only does this theory require a very different relation between 

 the hill and the roads, but it leaves unexplained the very even 

 width of the roads themselves. 



As loose matter would be, and in fact has been, washed down 

 much more abundantly in some places than in others, the roads 

 must, on any such hypothesis, vary greatly in width. This, how- 



* Op. cit. p. 255. 



